On 5/10/22 4:31 PM, Minchan Kim wrote:
+ int __mt = get_pageblock_migratetype(page);
+ int mt = __READ_ONCE(__mt);
Although I saw the email discussion about this in v2, that discussion
didn't go far enough. It started with "don't use volatile", and went
on to "try __READ_ONCE() instead", but it should have continued on
to "you don't need this at all".
That's really what I want to hear from experts so wanted to learn
"Why". How could we prevent refetching of the mt if we don't use
__READ_ONCE or volatile there?
Because you don't. There is nothing you are racing with, and adding
__READ_ONCE() in order to avoid a completely not-going-to-happen
compiler re-invocation of a significant code block is just very wrong.
So let's just let it go entirely. :)
Yeah, once it's clear for everyone, I am happy to remove the
unnecessary lines.
+
+ if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
With or without __READ_ONCE() or volatile or anything else,
this code will do what you want. Which is: loosely check
for either of the above.
What functional problem do you think you are preventing
with __READ_ONCE()? Because I don't see one.
thanks,
--
John Hubbard
NVIDIA
